A mechanical pencil with a lead magazine is already known from the DT-OS No. 21 53 400 and the corresponding U.S. Pat. No. 3,864,046 in which an automatic advance of the lead during use of the pencil as well as an automatic feed of leads from the lead magazine is to be carried out. For this purpose this mechanical pencil is provided with clamping tongs immovably held in axial direction, which in their rest position are open and which permit in this position a free advance of the lead from the lead magazine up to the inlet edge of an entrainment element. During writing a rear stroke of a lead tube is caused until the tension of a compression spring loading the lead tube is increased in such a manner that the clamping tongs are closed by means of a cone surface of a bushing, against which a compression spring abuts, preventing thereby a further inward movement of the lead. Subsequently thereto only the lead tube is further inwardly moved, whereby the entrainment element is moved over the lead. If now no load is exerted on the lead tube, the entrainment element moves along the lead, which is held by the clamping tongs. With this known construction it is, therefore, not possible to bring the lead into writing position only by loading and unloading the lead tube. Even if the lead is moved by hand into the lead tube so that the point of the lead is located at the outlet opening of the lead tube, then the lead tube together with the lead will carry out during loading a return stroke up to the clamping point of the tongs. During release of the lead tube the latter will then be forwardly moved, whereas the lead during part of this movement is still held by the clamping tongs so that the lead will remain behind the outlet opening of the lead tube. At subsequent loadings during use of the pencil, the lead will, therefore, travel further into the pencil. The function of an automatic advance of the lead is, therefore, not assured.
The U.S. Pat. No. 3,424,535 discloses a mechanical pencil in which advance of the lead during the use of the pencil, as well as advance of stored leads is to be caused automatically. To first insert a lead from a lead magazine, a spring loaded lead tube is loaded the same way as during writing and pushed into the housing of the pencil. A compression spring abuts against two clamping elements which are guided in a guide bushing. The clamping elements have conical clamping surfaces against which balls abut which are arranged in bores of the guide bushing, whereby the axial position of the clamping elements relative to the guide bushing remains fixed, until this unit is pushed so far back by the compression spring that the balls may escape into a lateral annular space. In this position the clamping elements are supposed to move radially away from each other, so that a lead from the lead magazine may move therebetween. A mechanism for this purpose is, however, not disclosed in this patent. If a lead is moved by hand between the clamping elements and the guide bushing together with the clamping halves are moved forwardly, while the lead tube is released, the balls move again into the clamping position. During renewed inward pressure of the lead tube, the balls remain, due to the presence of the lead, arrested in the clamping elements. During release of the lead tube, the lead is, however, not moved forwardly together with the same since the compression spring holds, on the one hand, the clamping elements in clamping position, but on the other hand, pushes the lead tube during release of the same forwardly.
The U.S. Pat. No. 2,911,948 discloses a mechanical pencil in which a lead tube, movable into the same, is fixedly connected with a clamping mechanism which, during use of the pencil, is released to slide rearwardly over the lead, whereas a second further inwardly located clamping mechanism holds the lead against rearward movement. During release of the lead tube, the lead will be moved by the first-mentioned clamping mechanism forwardly, whereas the second-mentioned clamping mechanism releases the lead. The disadvantage of this construction is that the clamping mechanisms, which are constructed as discs, each provided with a central hole, exert shearing forces onto the lead which, during writing, lead to a breaking of the latter. This construction has, therefore, not worked out in practice.